Centrifuge machines of the type used for rotating small parts, such as integrated circuit chips attached to a lead frame, typically utilize a rotatable drum. The drum has an interior cavity, usually of cylindrical shape, which is rotatable about its major axis. As is depicted in FIG. 1, a part 10 to be rotated is shown in a fixture 12 which is specifically adapted to receive the part.
The fixture 12 is removably attached to a centrifuge boat 14 thereby permitting various fixtures to be used interchangably with the same boat. The fixture 12, as shown, includes a cavity 16 for an integrated circuit (IC) chip which is mounted to a lead frame and enclosed to form an IC package 10. A magnet 18 is embedded in the fixture 12 so that when the IC package is placed in the fixture, the metal cover of the IC package will be adjacent the magnet. This magnet firmly holds the IC package in place during set up of the centrifuge machine and while other manual operations are being performed by the operator. The fixture 12 is typically fabricated from metal and requires delicate and highly accurate machining operations in its manufacture.
Two problems are encountered in the use of such metal fixtures. The first is that the metal fixture adds significant mass to the rotating component of the centrifuge machine. This results in the need for a larger, more expensive machine than would otherwise be required. The second problem is that the cavity 16, which is adapted to receive the IC package, has hard, non-yielding metalic surfaces which are flat. Part of the IC package on the other hand, is made of a very hard refractory material usually having uneven surfaces. These uneven surfaces are necessarily placed in contact with the metalic surfaces of the fixture resulting in actual contact only at spaced points or areas leaving other areas of the IC package unsupported. When the IC package and fixture are subjected to relatively high G-forces, e.g. over 30,000, during the course of operation of the centrifuge machine, the refractory material will sometimes crack thereby destroying the chip's usefulness.
To alleviate these problems associated with metal fixtures, wax is sometimes used in place of metal. In this case molten wax is poured into a cavity formed in the boat and the IC package impressed into the soft wax. Cracking is reduced by using this technique, however, some cracking will still occur due to air bubbles, being trapped in the wax resulting in uneven support of the IC package. Further, the process of making the wax fixture and subsequent cleaning of the IC package after the centrifuge operation is time consuming and tedious. Unfortunately, the wax fixture is necessarily damaged when removing the IC package and, therefore, is not reusable. The cleaning operation must be thorough to assure that all traces of wax contaminants are removed from the leads prior to assembling the IC package to a PC board.
The present invention overcomes these problems inherent in the metal and wax fixtures through the novel use of a moldable material to produce a reusable centrifuge fixture that is easily and inexpensively made.